Presently there is no simple way to use a mobile device to pay for a transaction. Use of specialized hardware to scan a barcode displayed on the mobile device or identify a specific code for a mobile device with a wireless signal such as Near Fields Communications (NFC) generally require a merchant to modify existing equipment. Merchants may be reluctant to adopt mobile-device payments if doing so requires additional expense. However, as smart phones and other mobile devices are becoming more ubiquitous consumer expectations for these devices are increasing and consumers may expect to use the mobile device itself for payment rather than a check or credit card. Although most merchants have point-of-sale (POS) devices for processing credit card, check, and cash transactions, a secure way of using a mobile device to pay for such transactions, without expensive modifications to existing POS devices, does not yet exist. Providing such an option would increase convenience for consumers without burdening merchants.